1. Field of the Invention
The present invention relates in general to a radio transmitting and receiving apparatus, and more particularly to a transmitting and receiving apparatus for a headphone of the digital type.
2. Description of the Prior Art
With reference to FIG. 1, there is shown a block diagram of a conventional transmitting apparatus for a headphone of the analog type. The illustrated transmitting apparatus comprises a first amplifying circuit 33 including amplifiers 31 and 32 for amplifying respectively a left sound signal L/S and a right sound signal R/S from a headphone output of a television set or a radio set by a predetermined amplification degree, a frequency modulating circuit 35 including frequency modulators 35a and 35b for frequency-modulating respective output signals from the amplifiers 31 and 32 in the first amplifying circuit 33, a first band-pass filtering circuit 36 including band-pass filters 36a and 36b for filtering respective output signals from the frequency modulators 35a and 35b in the frequency modulating circuit 35 to pass only frequency components of desired band width, and a transmitting circuit 37 including a transistor Q4 and a light-emitting diode D5, for transmitting output signals from the band-pass filters 36a and 36b in the first band-pas filtering circuit 36 with infrared rays carrying the signals.
With reference to FIG. 2, there is shown a block diagram of a conventional receiving apparatus for a headphone of the analog type. The illustrated transmitting apparatus comprises a circuit 38 including a transistor Q5 and a light-receiving diode D8, for receiving light signals transmitted from the transmitting circuit 37 in the transmitting apparatus and amplifying the received signals by a predetermined amplification degree, a second band-pass filtering circuit 41 including band-pass filters 39 and 40 for filtering respective output signals from the receiving circuit 38 to pass only frequency components of desired band width, a demodulating circuit 44 including demodulators 42 and 43 for demodulating respective output signals from the band-pass filters 39 and 40 in the second band-pass filtering circuit 41, a second amplifying circuit 47 including amplifiers 45 and 46 for amplifying respective output signals from the demodulators 42 and 43 in the demodulating circuit 44 by a predetermined amplification degree, and a sound output circuit 52 including a switch SW1, resistors R3 and R4, power amplifiers 48 and 49 and headphone speakers 50 and 52, for outputting analog sound signals from the amplifiers 45 and 46 in the second amplifying circuit 47 as sound signals.
The operation of the conventional transmitting and receiving apparatus for a headphone of the analog type which is constructed as mentioned above will be described.
Generally, the left sound signal L/S and the right sound signal R/S through the headphone output of the television set or radio set are weak in level. For this reason, in the transmitting apparatus, the left sound signal L/S and the right sound signal R/S are amplified by a predetermined amplification degree, respectively, by the amplifiers 31 and 32 in the first amplifying circuit 33. These amplified signals from the amplifiers 31 and 32 are frequency-modulated, respectively, by the frequency modulators 35a and 35b in the frequency-modulating circuit 35 and the frequency-modulated signals from the frequency modulators 35a and 35b are then applied to the band-pass filters 36a and 36b in the first band-pass filtering circuit 36.
Upon receiving the frequency-modulated signals from the frequency modulators 35a and 35b, the band-pass filters 36a and 36b filter, respectively, the received signals to pass only frequency components of desired band width. Then, the output signals from the band-pass filters 36a and 36b are transmitted to the light-receiving diode D8 in the receiving circuit 38 in the receiving apparatus through the light-emitting diode D5 in the transmitting circuit 37.
At this time, a diode D7 emits a light signal which is indicative of signal transmission. An LED driver 34 functions to drive the diode D7.
In the receiving apparatus, the signals inputted through the light-receiving diode D8 in the receiving circuit 38 are amplified, respectively, by a predetermined amplification degree by the transistor Q5 in the receiving circuit 38. Then, the band-pass filters 39 and 40 in the second band-pass filtering circuit 41 filter the amplified signals, respectively, from the receiving circuit 38 to pass only frequency components of desired band width. In the demodulating circuit 44, the demodulators 42 and 43 demodulate the output signals from the band-pass filters 39 and 40, respectively. Then, the demodulated signals are amplified by the amplifiers 45 and 46 in the second amplifying circuit 47. In the second amplifying circuit 47, the resistor R1 and the capacitor C1, and the resistor R2 and the capacitor C2 operate, respectively, as low-pass filters. In the sound output circuit 52, the amplified left and right signals from the amplifiers 45 and 46 are inputted to the power amplifiers 48 and 49 in mono or stereo in accordance with a selection of the switch SW1. Upon receiving the amplified left and right signals from the amplifiers 45 and 46, the power amplifiers 48 and 49 amplify the received signals, respectively, by a given amount and output the amplified signals as sound signals through the headphone speakers 50 and 51.
However, the conventional transmitting and receiving apparatus for a headphone of the analog type has a disadvantage, in that the apparatus is susceptible to noise. For this reason, when the apparatus transmits the signals utilizing an infrared ray, a malfunction thereof may occur according to a direction of the TV set or radio set. Also, there may occur interference due to an infrared ray signal from other systems.